Long structural elements such as columns are utilized to vertically support other members such as a floor and must be capable of withstanding compressive, buckling, and impact loads. These structural elements must support the load of the structure it supports and added loads such as vehicles and people.
The load bearing capabilities of these structural elements is determined by the shape of their cross-section, length and material. For lighter weight applications, C or L-channels and hollow structural beams such as round, square or rectangular tubing of aluminum or composite materials are often used. Although strong by design, one disadvantage of these conventional support columns and stanchions is their lower energy absorbing properties during a failure. In crash conditions where the structure may be in compression, composite materials often buckle suddenly and absorb little energy. Failure of these structural elements, known as compressive failure, occurs when the structural elements experience ultimate compressive stresses that are beyond what the material is capable of withstanding. Additionally, buckling failure may result from column instability as a function of the height and width of the structural element.
Composite materials having multiple layered materials are often used in structural applications due to their inherent strength properties. These materials also provide great design flexibility due to the large various material selections and the ability to create various shapes. Structural composite elements often are composed of sandwich-type structure having an outer skin and a filler or core material. The outer skin, or shell, defines the shape and structure of the element whereas the filler material supports the shell.
Various materials are used for both the skin and core of a structural element and are typically chosen based upon the application and environment of use. For example, weight is often a design consideration in aerospace applications. Structural elements such as support beams of wall structures often need to be both lightweight and capable of carrying a mechanical load. In these applications, lightweight metallic skins made of aluminum and nonmetallic skins of thin carbon/epoxy or graphite/epoxy skins are often utilized. Environmental factors such as temperature and corrosive conditions are also considered when selecting skin materials.
The filler material provides both structural support and maintains the shape of the composite element. A variety of filler materials are known and range from the simple, such as balsa wood or other metallic and nonmetallic stiffeners, to complex structures such as aluminum or other nonmetallic honeycomb cores. Core materials are selected based upon their material properties such as flexibility, stiffness and strength-to-weight ratios, energy and sound absorption properties and others.
Environment must also be considered when selecting filling materials. It is not uncommon for carbon/epoxy skinned materials to absorb water, especially in aircraft applications where the structure undergoes pressurizing and depressurizing and are constantly exposed to changing atmospheric conditions. Water trapped in a composite may degrade the filler materials. Water may vaporize in warm conditions and even freeze at low temperatures and high altitudes and when undergoing these phase changes may damage the filler structure especially honeycomb-style core fillers, or disbond the filler from the skin.
Accordingly, there is a need for a lightweight composite structural element that has low structural weight, high structural strength and is capable of absorbing impact or crash energy and stabilizing long-column buckling that does not suffer from the problems and limitations of the prior art.